1. Field of the Invention
The present invention relates to a method of crystallizing an amorphous silicon film, and more particularly, to a crystallization method using sequential lateral solidification (SLS).
2. Discussion of Related Art
Polycrystalline silicon (p-Si) and amorphous silicon (a-Si) are often used as the active layer material for thin film transistors (TFTs) in liquid crystal display (LCD) devices. Since amorphous silicon (a-Si) can be deposited at a low temperature to form a thin film on a glass substrate, amorphous silicon (a-Si) is commonly used in liquid crystal displays (LCDs). Unfortunately, amorphous silicon (a-Si) TFTs have relatively slow display response times that limit their suitability for large area LCDs.
In contrast, polycrystalline silicon TFTs provide much faster display response times. Thus, polycrystalline silicon (p-Si) is well suited for use in large LCD device, such as laptop, computers and wall-mounted television sets. Such applications often require TFTs having field effect mobility greater than 30 cm2/Vs together with low leakage current.
A polycrystalline silicon film is composed of crystal grains having grain boundaries. The larger the grains and the more regular the grain boundaries, the better the field effect mobility. Thus, a silicon crystallization method that produces large grains, ideally a single crystal, would be useful.
One method of crystallizing amorphous silicon into polycrystalline silicon is sequential lateral solidification (SLS). SLS crystallization uses the fact that silicon grains tend to grow laterally from the interfaces between liquid and solid silicon. With SLS, amorphous silicon is crystallized using a laser beam having a magnitude and a relative motion that melts amorphous silicon such that the melted silicon forms laterally grown silicon grains upon re-crystallization.
FIG. 1A is a schematic configuration of a conventional sequential lateral solidification (SLS) apparatus, while FIG. 1B shows a plan view of a conventional mask 38 that is used in the apparatus of FIG. 1A. In FIG. 1A, the SLS apparatus 32 includes a laser generator 36, a mask 38, a condenser lens 40, and an objective lens 42, the laser generator 36 generates and emits a laser beam 34. The intensity of the laser beam 34 is adjusted by an attenuator (not shown) in the path of the laser beam 34. The laser beam 34 is then condensed by the condenser lens 40 and is then directed onto the mask 38.
The mask 38 includes a plurality of slits “A” through which the laser beam 34 passes, and light absorptive areas “B” that absorb the laser beam 34. The width of each slit “A” effectively defines the grain size of the crystallized silicon produced by a first laser irradiation. Furthermore, the distance between each slit “A” defines the size of the lateral grains growth of amorphous silicon crystallized by the SLS method. The objective lens 42 is arranged below the mask and reduces the shape of the laser beam that passed through the mask 38.
Still referring to FIG. 1A, an X-Y stage 46 is arranged adjacent to the objective lens 42. The X-Y stage 46, which is movable in two orthogonal axial directions, includes an x-axial direction drive unit for driving the x-axis stage and a y-axial direction drive unit for driving the y-axis stage. A substrate 44 is placed on the X-Y stage 46 so as to receive light from the objective lens 42. Although not shown in FIG. 1A, it should be understood that an amorphous silicon film is on the substrate 44, thereby defining a sample substrate.
To use the conventional SLS apparatus, the laser generator 36 and the mask 38 are typically fixed in a predetermined position while the X-Y stage 46 moves the amorphous silicon film on the sample substrate 44 in the x-axial and/or y-axial direction. Alternatively, the X-Y stage 46 may be fixed while the mask 38 moves to crystallize the amorphous silicon film on the sample substrate 44.
When performing SLS crystallization, a buffer layer is typically formed on the substrate. Then, the amorphous silicon film is deposited on the buffer layer. Thereafter, the amorphous silicon is crystallized as described above. The amorphous silicon film is usually deposited on the buffer layer using chemical vapor deposition (CVD). Unfortunately, the CVD method produces amorphous silicon with a lot of hydrogen. To reduce the hydrogen content the amorphous silicon film is typically thermal-treated, which causes de-hydrogenation, which results in a smoother surface on the crystalline silicon film. If the de-hydrogenation is not performed, the surface of the crystalline silicon film is rough and the electrical characteristics of the crystalline silicon film are degraded.
FIG. 2 is a plan view showing a substrate 44 having a partially-crystallized amorphous silicon film 52. When performing SLS crystallization, it is difficult to crystallize all of the amorphous silicon film 52 at once because the laser beam 34 has a limited beam width, and because the mask 38 also has a limited size. Therefore, with a large size substrate, the mask 38 is typically arranged in several times over the substrate, while crystallization is repeated for the various mask arrangements. In FIG. 2, an area “C” that corresponds to one mask position is defined as a block. Crystallization of the amorphous silicon within a block “C” is achieved by irradiating the amorphous silicon with the laser beam several times.
Crystallization of the amorphous silicon film will be explained as follows. FIGS. 3A to 3C are plan views showing one block of an amorphous silicon film being crystallized using a conventional SLS method. In the illustrated crystallization, it should be understood that the mask 38 (see FIGS. 1A and 1B) has three slits.
The length of the lateral growth of a grain is determined by the energy density of the laser beam, by the temperature of substrate, and by the thickness of amorphous silicon film (as well as other factors). The maximum lateral grain growth should be understood as being dependent on optimized conditions. In the SLS method shown in FIGS. 3A to 3C, the width of the slits is twice as large as the maximum lateral grain growth.
FIG. 3A shows an initial step of crystallizing the amorphous silicon film using a first laser beam irradiation. As described with reference to FIG. 1A, the laser beam 34 passes through the mask 38 and irradiates one block of an amorphous silicon film 52 on the sample substrate 44. The laser beam 34 is divided into three line beams by the three slits “A.” The three line beams irradiate and melt regions “D”, “E” and “F” of the amorphous silicon film 52. The energy density of the line beams should be sufficient to induce complete melting of the amorphous silicon film, i.e., complete melting regime.
Still referring to FIG. 3A, after complete melting the liquid phase silicon begins to crystallize at the interfaces 56a and 56b between the solid phase amorphous silicon and the liquid phase silicon. Namely, lateral grain growth of grains 58a and 58b proceeds from the un-melted regions to the fully-melted regions. Lateral growth stops in accordance with the width of the melted silicon region when: (1) grains grown from interfaces collide near a middle section 50a of the melted silicon region; or (2) polycrystalline silicon particles are formed in the middle section 50a as the melted silicon region solidifies sufficiently to generate solidification nuclei.
When the width of the slits “A” (see FIG. 1B) is larger than twice the maximum lateral growth length of the grains, the width of the melted silicon region “D,” “E,” or “F” is also larger than twice the maximum lateral growth length of the grains. Therefore, lateral grain growth stops when the polycrystalline silicon particles are formed in the middle section 50a. Such polycrystalline silicon particles act as solidification nuclei in a subsequent crystallization step.
As discussed above, the grain boundaries in directionally solidified silicon tend to form perpendicular to the interfaces 56a and 56b between the solid phase amorphous silicon and the liquid phase silicon. As a result of the first laser beam irradiation, crystallized regions “D,” “E,” and “F” are formed in one block. Additionally, solidification nuclei regions 50a are also formed.
As mentioned before, the length of lateral grain growth attained by a single laser irradiation depends on the laser energy density, the temperature of substrate, and the thickness of the amorphous silicon film. In the above-mentioned first laser beam irradiation, the grains generated by the lateral growth typically have a length generally ranging from 1 to 1.5 micrometers (μm).
FIG. 3B shows crystallizing the amorphous silicon film using a second laser beam irradiation. After the first laser beam irradiation, the X-Y stage or the mask 38 moves in a direction opposite to the lateral grain growth of the grains 58a or 58b (in FIG. 3A), i.e., an X-axial direction, by a distance of several micrometers, which is the same as or less than the maximum length of the lateral grain growth. Then, the second laser beam irradiation is conducted. During the second laser beam irradiation, the second laser beam irradiates portions of the grains 58a and a portion of amorphous silicon. The regions irradiated by the second laser beam are melted and crystallized as described above. The silicon grains 58a or/and the regions 50a generated by the first laser beam irradiation serve as seeds for the second crystallization. Thus the lateral grain growth proceeds in the second melted regions. Silicon grains 58c formed by the second laser beam irradiation continue to grow adjacent to the silicon grains 58a formed by the first laser beam irradiation, and silicon grains 58d grown from an interface 56c are also formed. The lateral growth of these grains 58c and 58d stops when the nuclei regions 50b are formed in a middle section of the silicon region melted by the second laser beam irradiation.
Accordingly, by repeating the foregoing steps of melting and crystallizing, one block of the amorphous silicon film is crystallized to form grains 58e as shown in FIG. 3C.
The above-mentioned crystallization processes conducted within one block are repeated block by block across the amorphous silicon film. Therefore, the large size amorphous silicon film is converted into a crystalline silicon film. While generally successful, the conventional SLS method described above has disadvantages.
Although the conventional SLS method produces relatively large size grains, the X-Y stage or the mask must repeatedly move a distance of several micrometers to induce lateral grain growth. Therefore, the time required to move the X-Y stage or the mask 38 takes up, a major part of the total process time. This significantly decreases manufacturing efficiency.